Adjustable resistor with slider made from elastomeric material

ABSTRACT

The invention relates to an adjustable resistor comprising a strip of resistance material arranged on an insulating carrier and a slider made of conductive material guidable along the strip and contacting the strip, wherein the slider is manufactured from elastomeric material. A slider manufactured from an elastomeric material will, as a result of its flexibility, make contact with the resistance strip over a relatively large part of the width of the resistance strip so flat the problems of an uneven current distribution associated with one-point contact are avoided. The use of an elastomeric material further provides the option of dimensioning and shaping the slider such that it can be handled easily by means of a positioning machine, and the problems of the usually elongate rigid sliders on the production line are avoided. The slider is preferably manufactured from conductive rubber or a conductive plastic. According to yet another embodiment the slider is clamped in a carriage movable parallel to the strip.

FIELD OF THE INVENTION

The present invention relates to an adjustable resistor comprising astrip of resistance material arranged on an insulating carrier and aslider made of conductive material movable along the strip andcontacting the strip.

BACKGROUND OF THE INVENTION

Such adjustable resistors are generally known, for instance in the formof potentiometers, wherein the slider is formed by an element usuallymade of copper or silver which is movable along a carbon strip. Suchadjustable resistors are also known in the form of for instance ceramicprinted circuit boards on which strips of resistance material arearranged along which a slider made of copper or silver is movable andwhich can be used in particular to adjust the speed of an electricalhand-tool. In both cases a slider of a rigid, though slightly resilientmaterial is moved along a resistance strip. The resilience of thematerial is necessary herein to press a contact surface of the slideragainst the resistance strip.

It is noted here that while such a material is resilient, it is hardlyflexible. This has the consequence that generally only one point of theslider is in contact with the resistance strip. A current can thus flowonly at this one point between the slider and the resistance strip. Thecurrent flowing through the resistance strip to the slider will,starting from one end of the resistance strip, initially be fullydistributed over the width of the resistance strip, but in the vicinityof the contact position of the slider will be distributed with a greatercurrent density to this contact position. This results in an unevencurrent distribution and thus to a locally higher current density and alocally greater resistance. The curve of the adjustable resistor is thusnot linear. This problem otherwise occurs both when the adjustableresistor is used purely as an adjustable resistor and when it is used asa potentiometer. In the case of a potentiometer, a current flowing fromthe one end of the resistance strip to the other end of the resistancestrip is superimposed on the said current.

U.S. Pat. No. 4,833,440 provides an adjustable resistor comprising astrip of resistance material arranged on an insulating carrier and aslider made of conductive material guidable along the strip andcontacting the strip, wherein the slider is manufactured fromelastomeric material.

This prior art resistor avoids the problems mentioned above.

However, this prior art resistor has the draw back that an unevendistribution of the current is created through the cross-section of theresistance strip in the vincinity of the slider, when the contactsurface between the resistance strip and the slider does not extend overthe resistance strip.

BRIEF DESCRIPTION OF THE INVENTION

The present invention aims to provide such a resistor, wherein thisproblem is avoided.

This aim is reached in that the resistance strip is divided into twoparallel extending strips of equal width, and that the contact surfaceof the slider is interrupted and that each of the thus formed parts ofthe contact surface makes contact with one of the strips.

The problems associated with the prior art are avoided by theconfiguration of the two resistance strips. The resistance cannot becomesmaller between the two sliders than the resistance of the fixedconnection there between.

A slider manufactured from an elastomeric material will, as a result ofits flexibility, make contact with the resistance strip over arelatively large part of the width of the resistance strip, so that theproblems of an uneven current distribution associated with one-pointcontact are avoided.

The use of an elastomeric material further provides the option ofdimensioning and shaping the slider such that it can be handled easilyby means of a positioning machine, and the problems of the usuallyelongate rigid sliders on the production line are avoided.

The slider is preferably manufactured from conductive rubber or aconductive plastic. According to yet another embodiment the slider isclamped in a carriage movable parallel to the strip.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be elucidated hereinbelow with reference tothe annexed drawings, in which:

FIG. 1 shows a schematic perspective view of a first embodiment of anadjustable resistor according to the present invention;

FIG. 2 shows a sectional view of the adjustable resistor depicted inFIG. 1;

FIG. 3 shows a sectional view of a variant of the adjustable resistordepicted in FIGS. 1 and 2;

FIG. 4 shows a sectional view of a second variant of an adjustableresistor according to the invention; and

FIG. 5 is a top view of resistance strips of the variant shown in FIG.4.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a base plate 1 which is manufactured from insulatingmaterial, for instance from ceramic insulating material. This supportplate serves as carrier for the components arranged thereon which, inaddition to a resistance strip 2 arranged thereon, generally alsoinclude other elements such as semiconductors. Such a carrier usuallyforms part of control electronics for adjusting the rotation speedand/or the torque of electrically driven hand-tools in which such asupport plate is arranged.

As stated, a strip 2 of resistance material is arranged on support plate1 This resistance material is formed by a material which includesconductive particles, for instance carbon, and the specific resistanceof which has a value such that the total resistance of the stripacquires the required value. On at least one side the resistance strip 2is connected to a conductive strip 3 manufactured from conductivematerial, for instance of copper or of silver. This strip forms theconnection to the other components of the circuit in question. In somecases the other side of the resistance strip is likewise connected tothe electronic circuit so that a potentiometer-like configuration isobtained. This is not usually the case however. Above support plate 1 isarranged a carriage 4 which is movable parallel to resistance strip 2 bymeans of guide means, for instance rails, not shown in the drawing.

The carriage 4 is usually connected to an element movable separately inthe relevant direction, for instance the trigger of a switch. Carriage 4has a sleeve form, wherein a slider 5 according to the invention isarranged in the hollow interior of the sleeve. This slider ismanufactured according to the invention from elastomeric material.Slider 5 has dimensions such that on its underside the slider contactsthe resistance strip 2, Slider 5 is herein provided on its undersidewith a contact surface 6 which is slightly curved in the plane parallelto the direction of movement and perpendicularly of the strip.

As stated, the slider is manufactured from conductive elastomericmaterial, for instance rubber. Conductive rubber is a type of rubberwhich has acquired a certain degree of conductivity through the additionof conductive particles. The resistance will of course be higher thanthat of the usual sliders made of copper or silver, but in view of theslightly larger dimensions and the larger cross-section of the slidermaterial this is generally no problem. The electronics can moreovercompensate the effects thereof.

In the above stated embodiment the width of the contact surface 6 of theslider is the same as or slightly larger than the width of resistancestrip 2. Assuming that the resistance of the resistance strip isdistributed linearly, a linear resistance change is detected duringreciprocal movement of the slider.

Various aspects are shown in cross-section in FIG. 2.

FIG. 3 shows an embodiment which has a curved slider 7 which is clampedat both its ends 8,9 in sleeves 11,12 arranged on a carriage 10. Herealso is a curved contact surface which, however, corresponds with theside surface of the slider and not with the end surface.

Shown in FIG. 4 is an embodiment wherein there are two separateresistance strips 13,14, as is also shown in FIG. 5. Slider 15 istherefore provided with two separate contact surfaces 16 respectively 17which are in contact with the associated resistance strip. Slider 15 isprovided for this purpose with a cut-away portion 18. This embodimentotherwise corresponds with the embodiment shown in FIGS. 1 and 2.

As Shown in FIG. 5, the two resistance strips are mutually connected onone end by means of a conductive path 19. The two resistance strips13,14 are connected in parallel by this configuration. Such aconfiguration is particularly advantageous when the contact surfacebetween resistance strip and slider does not extend over the full lengthof the resistance strip. In such a situation an uneven distribution ofthe current is created through the cross-section of the resistance stripin the vicinity of the slider, this being expressed in the form of anincrease in resistance. This increase in resistance is particularlynoticeable when the slider is situated in the vicinity of one of theends of the strip. The problems associated herewith are avoided by theconfiguration of two resistance strips. The resistance cannot becomesmaller between the two sliders than the resistance of the fixedconnection therebetween.

As shown in FIGS. 2 and 3, a layer of lubricant 30 (not necessarilyshown to scale) may cover the contact surface of a slider 5 or 7.

It will be apparent that it is possible to deviate in various ways fromthe shown embodiments without departing from the inventive concept. Itis thus possible for instance to apply the invention on singlepotentiometers, for instance multilayer potentiometers. These multilayerpotentiometers generally have a great accuracy which is reduced by theeffect of the one-point contact with the slider. This inaccuracy isavoided by using a slider made from an elastomeric material whichextends over a substantial part of the width of the resistance strip.

What is claimed is:
 1. An adjustable resistor, comprising resistancematerial arranged on an insulating carrier, said resistance materialdivided into two parallel extending strips of equal width, and a slider,wherein said slider comprises conductive elastomeric material having aninterrupted portion forming two contact surfaces, said slider isguidable along the strips with each contact surface contacting one ofthe strips, and said strips and said slider are configured so as to formresistors in electrical parallel.
 2. An adjustable resistor as claimedin claim 1, wherein said conductive elastomeric material is conductiverubber or conductive plastic.
 3. An adjustable resistor as claimed inclaim 2, wherein the slider is clamped into a carriage movable parallelto the strips.
 4. An adjustable resistor as claimed in claim 2, whereinat least one of said contact surfaces makes contact with one of thestrips over the strip's full width.
 5. An adjustable resistor as claimedin claim 1, wherein the slider is clamped into a carriage movableparallel to the strips.
 6. An adjustable resistor as claimed in claim 5,wherein the carriage comprises a sleeve enclosing the slider and inwhich an end of the slider is fixedly clamped.
 7. An adjustable resistoras claimed in claim 5, wherein at least one of said contact surfacesmakes contact with one of the strips over the strip's full width.
 8. Anadjustable resistor as claimed in claim 5, wherein the strips includeends and are mutually connected on at least one pair of their ends. 9.An adjustable resistor as claimed in claim 5, wherein the adjustableresistor forms part of a circuit for adjusting the rotation speed of anelectrical hand-tool.
 10. An adjustable resistor as claimed in claim 1,wherein at least one of said contact surfaces makes contact with one ofthe strips over the strip's full width.
 11. An adjustable resistor asclaimed in claim 10, wherein the strips include ends and are mutuallyconnected on at least one pair of their ends.
 12. An adjustable resistoras claimed in claim 10, wherein the adjustable resistor forms part of acircuit for adjusting the rotation speed of an electrical hand-tool. 13.An adjustable resistor as claimed in claim 1, wherein at least one ofthe contact surfaces is rounded.
 14. An adjustable resistor as claimedin claim 1, wherein the strips include ends and are mutually connectedon at least one pair of their ends.
 15. An adjustable resistor asclaimed in claim 14, wherein the adjustable resistor forms part of acircuit for adjusting the rotation speed of an electrical hand-tool. 16.An adjustable resistor as claimed in claim 1, wherein the adjustableresistor forms part of a circuit for adjusting the rotation speed of anelectrical hand-tool.
 17. An adjustable resistor as claimed in claim 1,wherein a lubricant layer covers at least one contact surface.